FIG. 1 (which is taken from U.S. Pat. No. 4,688,160) shows an example of a forward power converter comprising a dc input 101, 102 coupled to the primary winding 109 of a transformer 110. The primary winding 109 is connected in series with a switching device 105, here a bipolar transistor, which switches on and off, during an on period building up magnetising flux in the primary winding 109, which drives a current in a secondary winding 111 of the transformer. Unlike a so-called flyback converter, in a forward converter the primary and secondary windings have matched polarities, as indicated by the dots on the windings in FIG. 1. The output from the transformer 110 is rectified by a rectifier 114 and smoothed by a smoothing capacitor 119 to provide a dc output 121, 122. When switch 105 is off the core of the transformer is “reset” allowing the magnetising flux to return to its initial state. In the example of FIG. 1 (U.S. Pat. No. 4,688,160) this is performed by resonant action between the magnetising inductance of transformer 110 and a capacitor 113 shunting diode 114, returning energy to the input voltage source.
The circuit of FIG. 1 includes a large output choke 117 between rectifier 114 and smoothing capacitor 119, and a freewheeling or “flyback” diode 115 across the series combination of choke 117 and smoothing capacitor 119. This is because when the switch 105 is turned off, because the primary and secondary windings have the same sense, rectifier 114 immediately becomes non-conducting. The function of the freewheeling diode 115 is to allow the choke 117 to maintain a continuous output current into output node “X” when switch 105 is off by providing a path for this current.
FIG. 1 shows a conventional, continuous forward converter. There are many other prior art documents describing such converters, including, for example, U.S. Pat. Nos. 4,415,959; 6,760,236; 6,304,463; 6,252,781; and the reference design SLUA276 for the Texas Instruments UCC38C42. In some of these later circuits the secondary side diodes are replaced by synchronous rectifiers embodied in MOS transistors. Other background prior art can be found in U.S. Pat. No. 4,788,634 which describes a resonant forward converter in which natural self-inductance of the transformer in parallel with the transformer provides a resonant “ring” so that the switching circuit can be self-resonant; and U.S. 2005/0270809 (WO 2004/057745) which describes use of an auxiliary transformer in a current limiting circuit.
The inventors have recognised that improved operation, such as improved regulation and start-up may be achieved by use of switch control in a discontinuous current flow mode.